1. Field of the Invention
The present invention relates to a magnetic resonance installation of the type having a basic field magnet, a number of gradient field coils, an RF transmission and reception unit and a control unit for actuating the gradient field coils and the RF transmission and reception unit for performing magnetic resonance measurements. The invention also relates to a method for operating such a magnetic resonance installation.
2. Description of the Prior Art
Magnetic resonance installations of the above general type are used in medical diagnosis in order to record images of the inside of a patient's body. Thus, magnetic resonance installations for imaging can be used, for example, in neurology, angiography or cardiology.
A frequent field of application for magnetic resonance tomography is the visualization or monitoring of tumors in cancer treatment. In the case of a recent technique for handling such tumors, chemotherapy or radiation treatment is supported or replaced by targeted heating of the tumor-containing region by means of focused irradiation with radio frequency (RF) energy. This recent technique is known by the term selective hyperthermia. With currently available hyperthermic appliances, the patient is positioned in a hyperthermic applicator, so that the region of the body that is to be treated is arranged approximately in the center below the applicator. The hyperthermic applicator is composed of a number of RF dipoles which are arranged in array form and are each supplied with pulsed or continuous RF power of defined amplitude and phase. The phase and amplitude of the radio frequency on each individual dipole are chosen such that the location of the region which is to be treated, i.e. of the tumor, is superimposed with the RF energy radiated from the individual dipoles such that the maximum field strength is achieved at that point. Some of the focused RF energy is absorbed by the tissue in the region of the tumor, so that this region is heated as a function of the radiated RF energy. Since the tumor-containing tissue is more heat sensitive than healthy tissue, the heating damages it to a greater extent than the surrounding healthy tissue. Such targeted heat treatment can cause the tumor-containing tissue to die.
A fundamental problem in hyperthermic treatment is the different propagation speed of electromagnetic waves in the tissue and in the surrounding air. Depending on the anatomy of the patient, the propagation path of the electromagnetic waves from the transmission dipoles to the tumor is filled by tissue or air to a greater or lesser extent. This influences the focusing, however, which means that it has not always been possible hitherto to focus the RF energy in optimum fashion during hyperthermic treatment without further auxiliary means. In the case of currently available hyperthermic appliances, the gap between the patient and the applicator is therefore filled with a water cushion which is filled with a special water solution after the patient has been positioned. This water cushion approximately aligns the propagation speeds of the RF radiation in the patient's body and between the body and the hyperthermic applicator, so that sufficiently good focusing is achieved even for different patient anatomies. However, this procedure is an unpleasant experience particularly for claustrophobic patients. In addition, simultaneous application of other applicators, for example for physiological monitoring of the patient during the hyperthermic treatment, is made more difficult by the water cushion, since little space remains for positioning additional applicators.
Hyperthermic treatment also requires the tissue temperature to be monitored during the treatment. This is currently achieved using special temperature sensors which are mounted on catheters. During the treatment, the catheters are inserted with the temperature sensors through the patient's skin and are taken to the irradiated tissue. This invasive method, however, puts an additional strain on the patient.
The search for improved techniques for recording the tissue temperature during hyperthermic treatment also takes the use of magnetic resonance measurements into consideration. The approach pursued in this context involves determining the temperature by means of a magnetic resonance examination which runs at the same time as the hyperthermic treatment. To this end, the hyperthermic applicator is placed in the examination space of a magnetic resonance installation and a magnetic resonance measurement is performed at the same time as the heating. The temperature of the tissue can be derived from the T1–T2 shift in the magnetic resonance signals obtained from the body region of interest.
One problem when applying a new approach to temperature measurement is the accuracy of the temperature measurement. This accuracy is currently barely sufficient, since the hyperthermic applicator is arranged between the RF transmission and reception unit of the magnetic resonance installation and the patient, which means that the received signal from a magnetic resonance echo is received only very weakly by the magnetic resonance installation's RF transmission and reception unit. In addition, the magnetic resonance signal is attenuated by the water cushions arranged between the hyperthermic applicator and the patient. Another cause of the insufficient accuracy of such temperature measurement is the choice of RF transmission frequency in the magnetic resonance installation. These magnetic resonance frequencies need to be sufficiently separated from the radio frequency of the hyperthermic applicator in order to decouple the magnetic resonance system from the hyperthermic system and to avoid mutual interference by the two systems. Known hyperthermic applicators operate in the frequency range of 100 MHz in order to achieve sufficient focusability for the radio frequency field in the patient's body. For this reason, the magnetic resonance frequencies are usually chosen in a range of 8–64 MHz in order to keep a sufficient separation from the 100 MHz of the hyperthermic applicator. To excite the magnetic resonance, however, the chosen magnetic resonance frequencies require magnetic field strengths in the basic field magnet of between 0.2 T and 1.5 T. At such basic field strengths, the temperature-dependent T1 T2 shift is not very distinct, however, which means that this also impairs the accuracy of the temperature determination.